Conventional blow molding apparatus are typically provided with a plurality of identical molds and are capable of in-line production of identical bottles. One type of blow molding apparatus is a wheel type blow molding apparatus comprising a wheel having a plurality of identical molds positioned about the circumference thereof. Each mold typically includes two mold halves each comprising a mold cavity half therein so that when the mold halves are closed the mold defines a mold cavity corresponding to the configuration of the article to be molded, such as a bottle. When employed for extrusion blow molding, the wheel will rotate each mold, seriatim, to an extrusion die positioned adjacent to the wheel. As each mold approaches the die, a parison having predetermined characteristics is extruded from the die and the mold halves of the approaching mold close around, and take hold of, the parison. The wheel then advances the mold and parison to a blowing needle which introduces internal pressure to the parison forcing it to expand to the configuration of the mold cavity and thus forming the desired bottle. The bottle is then cooled and released from the mold.
When exceptionally large production is required, multiple wheel type blow molding apparatus may be employed simultaneously to increase production output. Large production runs allow for continuous wheel operation at full capacity and minimal wheel downtime. Economies of scale and reduced costs per bottle are achieved.
It is occasionally necessary to manufacture only a small production run of identical articles. Short run production jobs, however, are less economically advantageous. The costs associated with such production runs (e.g., creating multiple molds, testing and wheel downtime for mold changeover) are spread over fewer bottles resulting in a per bottle cost which is often prohibitive. Operating a wheel at less than full capacity or using only a few molds on a wheel is possible to accomplish a short run, but such alternatives also increase per unit bottle cost. Accordingly, conventional wheel type blow molding machines are disadvantageous for short run production, ultimately making short run business unattractive.
U.S. Pat. No. 5,556,648 to Budzynski teaches providing each mold of a wheel type blow molding apparatus with a first and second inner cavity wherein the first cavity may be different from the second cavity, allowing for production of two distinct bottle configurations per mold. However, each mold on the wheel of Budzynski is identical to each of the others. Thus, the same number of bottles of each configuration will always be produced by the Budzynski apparatus and short run orders can not therefore be accommodated without replacing each mold on the wheel or operating the wheel at less than full capacity. Indeed, the resulting production of a wheel employing these molds would be identical to simultaneously running two wheels, each comprising a different set of identical molds.
Accordingly, there is a need for an improved method and apparatus for blow molding plastic articles for producing a short run of an article without significant downtime for testing and mold changeovers. There is also a related need for an improved method and apparatus for blow molding which reduces the costs associated with short run production jobs.